Fused terminal for use with a network protector

Details Fused terminal for use with a network protector Fused terminal for use with a network protector

2000-01-10

2002-03-19

Wong, Peter S. (Department: 2838)

Electricity: electrical systems and devices

Safety and protection of systems and devices

High voltage dissipation

Reexamination Certificate

active

06359765

ABSTRACT:

BACKGROUND OF INVENTION
The present invention is directed to power distribution connectors and, more particularly, to a terminal for a network protector having an integral fuse.
In New York, and in other major cities throughout North America, electricity is distributed from a utility company to customers via an electrical network; an electrical network being the most reliable, but also the most expensive method of electrical distribution. The electrical network generally consists of utility company high voltage sub-stations and distribution wiring, generally consisting of a number of high voltage cables called primary feeders (e.g., 13,800 volts) running in ducts under the city's streets. Various devices and interfaces connect the utility company and a customer such as, for example, a step-down transformer to step-down the voltage level generated and provided by the utility company and to ensure continued electrical service to the customer. At various points along the street the utility taps off the primary feeders to a primary side of a transformer that steps down the high voltage to a secondary voltage that is output from a secondary side of the transformer and that may be used directly by the utility's customers. In the United States, this secondary voltage may be either 120/208 volts or 277/480 volts, depending on the application. The secondary sides of a plurality of transformers are tied together to form a single electrical grid from which individual cables feed the end user or utility customer (also referred to herein as a secondary-side network).
The major advantage of the grid configuration of the secondary-side network is reliability. Because a number of transformer secondaries are tied together, an electrical fault on one of the transformers or one of the high voltage feeders does not deprive the customer of electricity. The remaining transformers pick up the load while the fault is repaired. The major disadvantage is that, because the transformer secondaries are tied together, absent some form of sectionalizing device, a fault on any one of the primary feeders would have disastrous consequences. The current flowing into the secondary-side network via the undamaged feeders would back-feed out of the secondary-side network via the damaged feeder's transformer towards the fault. This would result in customer power outages.
To avoid back-feeds, a sectionalizing device called a network protector is placed at the secondary side of the transformer. The network protector operates as a large switch between the transformer and the secondary-side network. The network protector (i.e., the switch) is housed in a steel box commonly referred to as a network protector housing. When utilities place network protectors in underground vaults, they use a submersible network protector housing which is designed to exclude water from the interior of the housing, even if the housing is completely submerged.
The network protector automatically opens and remains opened when current would flow out of the secondary-side network. The network protector is also designed to close and remain closed when current is flowing into the secondary-side network. When the network protector is operating correctly, a fault on its primary feeder will cause the network protector to open. When all the network protectors connected to the damaged feeder open, that damaged feeder will be electrically isolated from the secondary-side network and the back-feed of current will be arrested. In the event the network protector should fail to operate due to mechanical malfunction, it is necessary to isolate the network protector and transformer from the secondary grid. Fuses are installed on the secondary-side network of the network protector such that if the network protector fails to open when necessary, the feeder can be grounded causing those fuses to blow.
A fault on the secondary-side network of the network protector will not cause the network protector to open. Despite this fact, such a fault can have damaging consequences. It can cause large quantities of current to flow into the secondary-side network towards the fault. The heat generated by these large quantities of current may reach a level at which the network protector or the transformer is severely damaged. Another function of the network protector fuses, discussed above, is to avoid this possibility. If current through the fuse reaches a certain level, these fuses will blow and damage will be averted.
Because the network protector is mounted between the transformer and the secondary-side network, current normally flows through the network protector from the transformer to the secondary-side network. The network protector is connected to the secondary-side network via a plurality of terminals that exit from the top of the network protector housing.
Network protectors have undergone a number of design changes from the time they were introduced (around the 1920's) to the present. The first relevant designs were created by General Electric (G.E.) and Westinghouse Electric in the 1930s. The G.E. and Westinghouse designs had a number of similarities. In both cases the network protector fuses were located inside the network protector housing. Locating the fuse within the housing provided for a compact design of the network protector. In some cities, utilities pay taxes based on the amount of property its equipment occupies (e.g., network transformers and network protectors). Since city real estate is relatively expensive, height is a critical consideration for any network protector design.
However, internal fuses radiate a significant amount of heat. As heat inside the network protector housing increases, the current-carrying capacity of the fuses decreases. In addition, there is no forced-air cooling within the network protector housing; only ambient cooling is provided by cooling fins on the outside of the network protector housing. The heat generated by the internal fuses thus represents a limitation on the current-carrying capacity of a network protector.
Internal fuses may also lead to damage to the network protector. The most common fuse design is a link of copper that is bolted into the network protector housing to form a path for current to flow between the network protector and the secondary-side network. In the center of the fuse, the width of the copper is significantly reduced. At a certain temperature (i.e., at a certain level of current flow), the thin section of the fuse melts thus breaking the connection between the network protector and the secondary-side network. Problems result when severe secondary faults result in a rapid rise in the quantity of current flowing through the network protectors. The fuses may then blow violently spraying molten copper throughout the interior of the network protector housing. In addition to being messy, on 277/480 volt network protectors, this can result in damage due to arcing in the network protector housing.
In the 1970s, Westinghouse Electric developed a network protector which attempted to eliminate the disadvantages of internal fuses. The Westinghouse CMD network protector placed the network protector fuses outside the network protector housing. The fuses were housed in small boxes that were mounted on the top of the network protector housing. The network protector terminals were then mounted above the fuse boxes. By placing the fuses outside of the network protector housing, Westinghouse eliminated both disadvantages of internal fusing. The heat of the fuses was removed from the housing, which allowed Westinghouse to increase the current capacity of the network protector. Westinghouse was thus able to manufacture a lighter, less expensive network protector while maintaining the same current capacity as the older designs. Because the fuses were removed from the network protector housing, Westinghouse had also eliminated the problem of splattering molten copper resulting from a violent blowing of the fuses.
While Westinghouse had eliminated the major weakness of internal fusing, it had al

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